msm: smd: Add edge-to-subsystem lookup call

Add support for looking up the name of a subsystem
given an SMD edge.

This enables moving the PIL subsystem naming to
platform data instead of hard coding it in SMD,
SMD PKT, and SMD TTY.

Change-Id: I5f9325baff6eb8f414efa67e136164181aed9be3
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>